The present invention relates to an oscillation circuit having a surface acoustic wave resonator disposed at least in a feedback path of an amplifier, and a radio communication device using the same.
Recently, an oscillation circuit using a surface acoustic wave resonator is used for configuring the oscillation circuit to have a frequency in, for example, the GHz range. However, a surface acoustic wave resonator has a power handling capability that is lower than that of a dielectric filter. This causes a problem in that a large power application accelerates the deterioration of a inter digital transducer which results in a fluctuation of oscillation frequency, a reduction of oscillation power output and finally no oscillation.
In order to solve this problem, a method for manufacturing a surface acoustic wave resonator with improved power handling capability has been proposed (for example, in Japanese Unexamined Patent Publication No.10-75141). That is, an electrode composed of aluminum or an aluminum alloy is formed on a piezoelectric substrate. A layer composed of another element having a diffusion coefficient that is equal to or greater than that of the aluminum is formed on at least a part of the electrode. Subjecting the layer composed of the another element to a heat treatment at a temperature higher than that at which the another element can diffuse leads the another element to diffuse in a thickness direction of the electrode so as to form the electrode composed of the aluminum alloy including the another element.
In addition, a frequency converter equipped with a local oscillation part with an amplifier provided to a succeeding stage of an oscillation circuit so as to ensure the desired output power has been proposed (for example, in Japanese Unexamined Patent Publication No.5-121949).
In addition, a quartz crystal oscillator where an oscillation inverter composed of a MOS-FET for oscillation is connected in parallel with a quartz crystal resonator has been proposed. This sets an on-resistance of the MOS-FET configuring the oscillation inverter so as to control a power applied to the quartz crystal oscillator (for example, in Japanese Unexamined Patent Publication No.10-173442).
In the conventional example described in the above-mentioned Japanese Unexamined Patent Publication No.10-75141, while the power handling capability of the surface acoustic wave resonator itself can be improved, there is the unsolved problem that a continuous oscillating condition in which a large power is applied shortens an elapsed time in which the continuous oscillating can be maintained. As a result, the continuous oscillating condition cannot be stably maintained for a long time.
Also, in the conventional example described in the above-mentioned Japanese Unexamined Patent Publication No.5-121949, while the output power can be increased since the oscillation output from the oscillator in the local oscillation part is amplified by an amplifier after multiplying by a quadrupling circuit to be output, there is the unsolved problem that no countermeasures enable the surface acoustic wave resonator configuring the oscillation circuit to improve the power handling capability and to extend life.
In addition, in the conventional example described in the above-mentioned Japanese Unexamined Patent Publication No.10-173442, including a resistor so as to suppress the power of the amplifier controls the power applied to the quartz crystal oscillator, however, there is the unsolved problem that it cannot be applied to the oscillation circuit using the surface acoustic wave resonator.
The present invention focuses on the unsolved problems of the above-mentioned conventional examples and aims to provide an oscillation circuit and radio communication device using the same that are capable of maintaining a continuous oscillation condition for a long time while suppressing the power applied to the surface acoustic wave resonator.